1. Field of the Invention
The invention relates to apparatus for surveillance and identification of aircraft along runways, taxiways and other prescribed pathways on the surface of an airport.
2. Description of the Prior Art
In the prior art much attention has been devoted to enroute navigation by electronic means, for commercial aircraft especially. Approach and landing aids have also received much attention, the best known of these being of the ILS (Instrument Landing System) type providing cross-pointer indication in the cockpit of an aircraft in both the localizer (azimuth), and glideslope (elevation) coordinates. Various radar systems, including the well-known GCA (Ground Controlled Approach) system have also been effectively used to track aircraft approaching for landing so that verbal instructions can be given, by radio, permitting a pilot to correct his landing approach in both azimuth and elevation in essentially zero visibility conditions.
It has been more recently recognized that an additional problem exists once the aircraft has landed, namely, the problem of ground or airport surface surveillance and identification. Especially in large metropolitan airports, weather conditions as benign as atmospheric haze may restrict visibility to the end of a relatively long runway and the related taxiways and approach ramps, even for an optimally located control tower. Still further, the presence of buildings and other structures about the perimeter of such metropolitan airports produces further difficulty in obtaining clear visual surveillance of the entire surface of an airport from the control tower.
One of the well-known approaches to airport surface traffic control has been the use of scanning radars operating at very high frequencies, such as the so-called "K" radar band. This is considered necessary in order to obtain adequate definition and resolution. An existing airport, ground, traffic control equipment of that type is known in the art as ASDE (Airport Surface Detection Equipment). As generally conceived such equipment provides surveillance only, no discrete identification of aircraft on the surface being available.
Among the other proposed systems for the purpose is a socalled Tower-Automated, Ground Surveillance (TAGS) which uses multilateration to interrogate aircraft transponders.
Still further, the so-called LOCAR (Localized Cable Radar) has been proposed, this system being described in U.S. Pat. No. 3,872,474.
High definition surface surveillance equipment, such as the so-called ASDE has other significant disadvantages. Among these is the need for a relatively high antenna tower and a relatively large rotating antenna system thereon. Large reflectors or other expedients are employed to obtain sufficiently narrow beams for the purpose. Shadowing due to structures, undesired reflections and unacceptably high absorption of the transmitted radar energy in the presence of hydrometeoric phenomena occurs due to the extremely high radar frequencies employed.
The so-called TAGS system, while it does provide identification, depends on a complex, centralized, multi-lateration technique requiring expensive and complicated timing, blanking and processing equipment. Additionally, surveillance is usually not provided over the entire airport surface because central locations providing unshadowed line-of-sight over the entire airport surface of interest are not generally available.
The LOCAR system described in the aforementioned U.S. Pat. No. 3,872,474, is a system with which the present invention is compatible structurally and functionally. The LOCAR as described in U.S. Pat. No. 3,872,474 provides no discrete aircraft identification per se but can be augmented to do so by the addition of the apparatus hereinafter described.
The manner in which the invention deals with the disadvantages of the prior art to provide a unique, low-cost, highly effective, airport surface surveillance and identification system will be evident as this description proceeds.